Considering that the field of application of these electric fans is that of climate control and cooling systems for installation in motor vehicles, it should be observed that the main aims of developing electric fans for this purpose are: low acoustic noise, limited energy consumption and reduced costs.
These requirements have led to the adoption of sine-wave c.e.m.f. brushless motors (AC brushless motors) driven by inverter capable of generating sine-wave currents and making obsolete the use of PWM six-step driven trapezoidal c.e.m.f. motors (more commonly known as DC brushless motors).
The sine waveform of the c.e.m.f. and of the related phase current minimizes active torque ripple (virtually zero), thus reducing mechanical vibrations and acoustic noise.
It is also known that it is possible to minimize current draw to generate a certain drive torque, thereby maximizing electromechanical conversion efficiency through optimum drive of AC brushless motors which are normally driven by current-controlled, impressed voltage inverters.
To obtain this type of drive, the static switches must change state in such a way that the polar axis of the rotor magnetic field remains at 90 electrical degrees to the polar axis of the magnetic field generated by the current circulating in the stator windings, whatever the torque supplied and the rotation speed.
To obtain information about the angular position of the rotor, relatively expensive devices are normally used, including absolute encoders or Hall effect sensors, integral with the stator and suitably positioned angularly, to detect the sine waveform of the magnetic energizing field along the periphery of the rotor.
The output signals generated by the sensors are then suitably decoded to drive the static switches in such a way as to keep the angular shift of 90 electrical degrees between the rotor and stator magnetic fields.
This type of drive requires the use of the above mentioned position sensors, whose cost is relatively high.
In an attempt to reduce the cost of drives, driving strategies that do not use sensors of this type have been developed.
These driving strategies are based on the consideration that if drive is optimum, the c.e.m.f. and the phase current are in phase and vice versa at each point in the operating field (torque, rotation speed, DC supply voltage).
Consequently, these driving strategies and drives, which have come to be known as “sensorless”, are based on the reading of electrical quantities (e.g. voltage at motor terminals or current circulating in motor windings) to detect the points where the c.e.m.f. and the current cross zero (zero crossings), calculate the relative phase between c.e.m.f. and current and implement appropriate methods of driving the inverter static switches which tend to keep the two quantities in phase.
One disadvantage of these methods lies in the fact that to detect the zero crossing of the c.e.m.f., that is to say, to read the sign of the c.e.m.f., the current flowing through the windings must remain zero long enough to enable the reading to be taken, which contrasts with the desired sinusoidal waveform of the current.
For the deviation from the ideal to have negligible effects, the length of the time interval during which the current remains zero must be reduced to the minimum and, to eliminate the distortion induced by the controlled phase current interruption, however brief, and the risks of not reading the desired signal, sophisticated algorithms are introduced to calculate the angular position of the rotor in real time: in practice, these algorithms are an integral part of field-oriented controls (FOC in the jargon of the trade) and require the use of sophisticated and expensive controllers with high processing capacity (known as DSP controllers in the jargon of the trade).